Abstract Zika virus (ZIKV) and the four-dengue virus (DENV) serotypes are emerging mosquito-borne flaviviruses that pose serious threats to human health. People exposed to primary DENV infections develop long-term serotype- specific neutralizing and protective antibodies (Abs). Individuals exposed to secondary DENV infections with a new serotype develop a novel class of cross neutralizing and protective Abs that are even effective against serotypes not previously encountered by the individual. While the molecular properties of type-specific human neutralizing Abs have been well-defined, little is known about the origin and properties of DENV cross- neutralizing Abs induced after secondary infection. In this project, we will use an established human challenge model of secondary DENV infection to test the hypothesis that secondary infections activate and expand memory B cells from primary infections to generate somatically mutated Abs that bind with high affinity to epitopes that are conserved between DENV serotypes (Specific Aim 1). We will also define the molecular specificity and functional properties of neutralizing Abs induced in US travelers with laboratory confirmed ZIKV infections. Building on our discoveries about primary Ab responses to DENVs, we propose that people exposed to ZIKV as a primary flavivirus infection develop type-specific neutralizing Abs that target quaternary structure envelope (E) protein epitopes displayed on the viral surface (Specific Aim 2). While repeat infections with different DENV serotypes induce durable cross DENV neutralizing Abs, it is unclear if this response can expand to neutralize ZIKV. By analyzing Ab responses in people exposed to secondary DENV infections and DENV immune individuals infected with ZIKV, we will determine the molecular mechanisms and structural features that promote or restrict the breadth of Ab cross-neutralization between the DENV sero-complex and ZIKV (Specific Aim 3). The importance of our studies is underscored by the disappointing results recently reported with tetravalent live attenuated DENV vaccines to induce balanced cross-protective immunity to the 4 DENV serotypes. Our studies are relevant to the successful design and evaluation of the next generation of safe and effective vaccines against emerging flaviviruses. Moreover, patterns of cross-neutralizing and protective immunity between dengue and Zika viruses may explain the explosive spread of ZIKVs in Latin America compared to Asia.